On the radial metallicity gradient and radial migration effect of the Galactic disk

Abstract

We study the radial metallicity gradient $\Delta[M/H]/\Delta R_g$ as a function of [Mg/Fe] and $|Z|$ with the help of a guiding radius based on the Apache Point Observatory Galactic Evolution Experiment and Gaia and then analyze the radial migration effect on the radial metallicity gradient and metallicity-rotation gradient between the Galactic thin and thick disks. The derived trend of gradient $\Delta[M/H]/\Delta R_g$ versus [Mg/Fe] shows a transition at [Mg/Fe] $\sim 0.18$ dex, below which the gradient is negative and varies a little as [Mg/Fe] increases; however, it changes sharply in [Mg/Fe] ranges of 0.16-0.18, above which the gradient increases linearly with increasing [Mg/Fe], being a positive value at [Mg/Fe]$\gtrsim 0.22$ dex. These positive gradients in the high-[Mg/Fe] populations are found at $|Z| < 0.8$ kpc, and there are nearly no gradients toward higher $|Z|$. By comparing the metallicity distributions, the radial metallicity gradients $\Delta[M/H]/\Delta R$ and the metallicity-rotation gradients between the total sample and $|R-R_g|<2$ kpc subsample (or $|R-R_g|>2$ kpc subsample), we find that, for the thick disk, blurring flattens the gradient $\Delta[M/H]/\Delta R$ and favors metal-poor high-eccentricity stars. These stars are responsible for the measured positive metallicity-rotation gradient of the thick disk.